If girl little (0-14 age) you have to say "девочка" in other situations you nave to say "девушка"

What Barbara said is true, in general. But in daily unformal conversation we often call females "девочки" or even less formal "девчонки", independently of their age. For example, in female friends' conversation you may hear: "На верчеринке были только девчонки" = "At the party there were only girls / women" (the age of those girls may be both 20 and 60).

Women like then you use word "девчонки" toward them. :)) Use it a lot.